Identify the italicized sound effect. On first looking into Chapman's Homer

Much have I travell'd in the realms of gold,
And many goodly states and kingdoms seen;
Round many western islands have I been
Which bards in fealty to Apollo hold.
Oft of one wide expanse had I been told [5]
That deep-brow'd Homer ruled as his demesne;
Yet did I never breathe its pure serene
Till I heard Chapman speak out loud and bold:
Then felt I like some watcher of the skies
When a new planet swims into his ken; [10]
Or like stout Cortez when with eagle eyes
He star'd at the Pacific–and all his men
Look'd at each other with a wild surmise–
Silent, upon a peak in Darien.

—John Keats

assonance
onomatopoeia
alliteration